This invention is generally directed to amorphous silicon imaging members, and more specifically, the present invention is directed to photoresponsive layered imaging members, or devices comprised of hydrogenated amorphous silicon and two overcoating layers of silicon nitrides. In one embodiment of the present invention there is provided a layered photoresponsive imaging member comprised of a supporting substrate, a blocking layer of hydrogenated amorphous silicon with dopants therein, a bulk photoconducting layer of hydrogenated amorphous silicon with optional dopants therein, a first overcoating layer of nonstoichiometric silicon nitride with excess silicon, and in contact therewith a second overcoating layer of near stoichiometric silicon nitride; that is for example, where the amount of silicon present is from between about 43 to 67 atomic percent. Further, in a specific embodiment of the present invention there is provided a layered photoresponsive imaging member comprised of a supporting substrate; a blocking layer of hydrogenated amorphous silicon with a high concentration, for example about 100 parts per million, of boron therein; a bulk photoconducting layer of hydrogenated amorphous silicon with a small amount of boron therein, for example 5 parts per million; a first overcoating layer of nonstoichiometric silicon nitride with excess silicon; and in contact therewith a second overcoating layer of near stoichiometric silicon nitride. These imaging members can be incorporated into electrophotographic, and in particular xerographic imaging and printing systems. Moreover, the imaging members of the present invention possess high charge acceptance values, that is for example, in excess of 40 volts/microns; and the members can be fabricated in a desirable thickness of from, for example, about 100 microns or less. Also, the imaging members of the present invention have desirable low dark decay properties. Further, the photoresponsive imaging members of the present invention, when incorporated into xerographic imaging and printing systems, are insensitive to humidity and ions generated from corona charging devices enabling these members to formulate acceptable images of high resolution for extended time period exceeding, in most instances, more than 100,000 imaging cycles. Also, the specific overcoating of the present invention eliminate the high undesirable lateral movement of charges at the interface between the photoconducting layer and the near stoichiometric silicon nitride overcoating thereby reducing band bending, a prior art problem; and thus enabling images with increased resolution and less print deletions.
Electrostatographic imaging and particularly xerographic imaging processes are well known, and are extensively described in the prior art. In these processes a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. The photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material, and in many instances, a thin barrier layer is situated therebetween to prevent charge injection from the substrate, which could adversely affect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium such as selenium-tellurium, selenium-arsenic, and the like. Additionally, there can be selected as photoresponsive imaging members various organic photoconductive materials including, for example, complexes of trinitrofluorenone and polyvinylcarbazole. Recently there have been disclosed layered organic photoresponsive devices with aryl amine hole transporting molecules, and photogenerating layers, reference U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Also known are hydrogenated amorphous silicon photoconductors, reference for example U.S. Pat. Nos. 4,265,991 and 4,225,222. There is disclosed in the '991 patent an electrophotographic photosensitive member comprised of a substrate, and a photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen and having a thickness of 5 to 80 microns. Additionally, this patent describes several processes for preparing amorphous silicon. In one process, according to the '991 patent there is prepared an electrophotographic photosensitive member which involves introducing a gas containing silicon and hydrogen atoms, providing an electrical discharge by electric energy to ionize the gas, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second by utilizing an electric discharge while maintaining the temperature of the substrate between 50.degree. C. to 350.degree. C. thereby resulting in an amorphous silicon photoconductive layer of a predetermined thickness. Although the amorphous silicon device described in this patent is photosensitive, after a minimum number of imaging cycles, less than about 1,000 for example, unacceptable low quality images of poor resolution with many deletions may result. With further cycling, that is subsequent to 1,000 imaging cycles and after 10,000 imaging cycles, the image quality may continue to deteriorate often until images are partially deleted.
There are also illustrated in copending applications photoconductive imaging members comprised of amorphous silicon. Accordingly, for example, there is illustrated in copending application U.S. Ser. No. 695,990, entitled Electrophotographic Devices Containing Compensated Amorphous Silicon Compositions, the disclosure of which is totally incorporated herein by reference, an imaging member comprised of a supporting substrate and an amorphous hydrogenated silicon composition containing from about 25 parts per million by weight to about 1 percent by weight of boron compensated with substantially equal amounts of phosphorous. Furthermore, described in copending application U.S. Pat. No. 4,544,617, entitled Electrophotographic Devices Containing Overcoated Amorphous Silicon Compositions, the disclosure of which is totally incorporated herein by reference, are imaging members comprised of a supporting substrate, an amorphous silicon layer, a trapping layer comprised of doped amorphous silicon, and a top overcoating layer. Also, nonstochiometric silicon nitride overcoatings for amorphous silicon imaging members are disclosed in the aforementioned copending application. Additionally, described in copending application U.S. Pat. No. 4,613,556 entitled Heterogeneous Electrophotographic Imaging Members of Amorphous Silicon, the disclosure of which is totally incorporated herein by reference, are imaging members comprised of hydrogenated amorphous silicon photogenerating compositions, and a charge transporting layer of plasma deposited silicon oxide. Moreover, there are disclosed in the prior art amorphous silicon photoreceptor imaging members containing, for example, stoichiometric silicon nitride overcoatings; however, these members in some instances generate prints of low resolution as a result of the band bending phenomena. Additionally, with the aforementioned silicon nitride overcoatings, the resolution loss can in many instances be extreme thereby preventing, for example, any image formation whatsoever.
Other representative prior art disclosing amorphous silicon imaging members, including those with overcoatings, are U.S. Pat. Nos. 4,460,669; 4,465,750; 4,394,426; 4,394,425; 4,409,308; 4,414,319; 4,443,529; 4,452,874; 4,452,875; 4,483,911; 4,359,512; 4,403,026; 4,416,962; 4,423,133; 4,460,670; 4,461,820; 4,484,809; and 4,490,453. Additionally, patents that may be of background interest with respect to amorphous silicon photoreceptor members include, for example, U.S. Pat. Nos. 4,359,512; 4,377,628; 4,420,546; 4,471,042; 4,477,549; 4,486,521; and 4,490,454.
Further, other representative prior art patents that disclose amorphous silicon imaging members include, for example, U.S. Pat. No. 4,357,179 directed to method for preparing imaging members containing high density amorphous silicon or germanium; U.S. Pat. No. 4,237,501 which discloses a method for preparing hydrogenated amorphous silicon wherein ammonia is introduced into a reaction chamber; U.S. Pat. Nos. 4,359,514; 4,404,076; 4,403,026; 4,397,933; 4,423,133; 4,461,819; 4,237,151; 4,356,246; 4,361,638; 4,365,013; 3,160,521; 3,160,522; 3,496,037; 4,394,426; and 3,892,650. Of specific interest are the amorphous silicon photoreceptors illustrated in U.S. Pat. Nos. 4,394,425; 4,394,426 and 4,409,308 wherein overcoatings such as silicon nitride and silicon carbide are selected. Examples of silicon nitride overcoatings include those with a nitrogen content of from about 43 to about 60 atomic percent.
Also, processes for depositing large area defect free films of amorphous silicon by the glow discharge of silane gases is described in Chittick et al., the Journal of the Electrochemical Society, Volume 116, Page 77, (1969). The fabrication and optimization of substrate temperatures during fabrication is illustrated by Walter Spear at the Fifth International Conference on Amorphous and Liquid Semiconductors presented at Garmisch Partenkirchen, West Germany in 1963. Other fabrication processes are described in the Journal of Noncrystalline Solids, Volumes 8 to 10, Page 727, (1972), and the Journal of Noncrystalline Solids, Volume 13, Page 55, (1973).
Moreover, illustrated in a copending application U.S. Ser. No. 781,858, entitled Overcoated Amorphous Silicon Imaging Members, the disclosure of which is totally incorporated herein by reference, is an imaging member comprised of a supporting substrate, a blocking layer of hydrogenated amorphous silicon containing dopants such as boron, a bulk photoconductive layer of hydrogenated amorphous silicon; and an overcoating layer of nonstoichiometric silicon nitride. One main advantage attributed to the photoresponsive imaging member of the present application in comparison to that referred to in the copending application is the provision of a more durable imaging member in view of the presence of a second top hard overcoating layer of near stoichiometric silicon nitride.
Although the above described imaging members, particularly those disclosed in some of the copending applications, are suitable for their intended purposes there continues to be a need for improved imaging members comprised of amorphous silicon. Additionally, there is a need for hydrogenated amorphous silicon imaging members that possess desirable high charge acceptance and low charge loss in the dark. Furthermore, there continues to be a need for improved hydrogenated amorphous silicon imaging members with a first overcoating layer of nonstoichiometric silicon nitride and a second top overcoating of near stoichiometric silicon nitride enabling the substantial elimination of the undesirable lateral motion of charge, and thereby permitting the generation of images of increased resolution. Furthermore, the imaging members of the present invention are more able to withstand the abrasive wear of developer materials as compared to amorphous silicon imaging members with only nonstoichiometric overcoatings of silicon nitride. Additionally, there continues to be a need for improved layered imaging members of hydrogenated amorphous silicon which are humidity insensitive, and are not adversely effected by electrical consequences resulting from scratching and abrasion. There is also a need for amorphous silicon imaging members which can be selected for use in repetitive imaging and printing systems. Furthermore, there is a need for hydrogenated amorphous silicon imaging members with low surface potential decay rates in the dark, and photosensitivity in the visible and near visible wavelength range. Further, there is a need for improved layered hydrogenated amorphous silicon which have very few image defects such as white spots with images of dark solids.